U.S. patent application number 12/745276 was filed with the patent office on 2010-12-09 for synthetic method for anti-oxidation ceramic coatings on graphite substrates.
This patent application is currently assigned to Changwon National University Industry Academy Cooperation Corps. Invention is credited to Yeon Gil Jung, Ji Hun Kang, Jeong-Pyo Kim, Sang Won Myoung.
Application Number | 20100310860 12/745276 |
Document ID | / |
Family ID | 41016250 |
Filed Date | 2010-12-09 |
United States Patent
Application |
20100310860 |
Kind Code |
A1 |
Jung; Yeon Gil ; et
al. |
December 9, 2010 |
SYNTHETIC METHOD FOR ANTI-OXIDATION CERAMIC COATINGS ON GRAPHITE
SUBSTRATES
Abstract
A method of forming an SiC or SiC/Si.sub.3N.sub.4 coating layer
on a bare graphite substrate via a solid-vapor process is
disclosed. Synthesis of the SiC coating layer on the graphite
substrate is accomplished by reaction of SiO vapor and carbon (C)
of the graphite, and that of the SiC/Si.sub.3N.sub.4 coating layer
is accomplished by reaction of SiO vapor, N.sub.2 and C of the
graphite. Thickness of the SiC coating layer is affected by
porosity of the graphite substrate, reaction temperature, and dwell
time. By controlling the reaction temperature, hardness of the SiC
coating may be increased to 10-15 times that of the bare graphite
substrate. The SiC/Si.sub.3N.sub.4 coating is much thinner than the
SiC coating and has a higher surface hardness. Thermal oxidation
tests show that the SiC or SiC/Si.sub.3N.sub.4 coated substrate
exhibits improved oxidation resistance over bare substrates. In
particular, the SiC/Si.sub.3N.sub.4 coated substrate shows
outstanding resistance to thermal oxidation.
Inventors: |
Jung; Yeon Gil;
(Changwon-si, KR) ; Myoung; Sang Won; (Masan-si,
KR) ; Kang; Ji Hun; (Hadong-gun, KR) ; Kim;
Jeong-Pyo; (Dongnae-gu, KR) |
Correspondence
Address: |
SCHMEISER, OLSEN & WATTS
22 CENTURY HILL DRIVE, SUITE 302
LATHAM
NY
12110
US
|
Assignee: |
Changwon National University
Industry Academy Cooperation Corps
|
Family ID: |
41016250 |
Appl. No.: |
12/745276 |
Filed: |
February 28, 2008 |
PCT Filed: |
February 28, 2008 |
PCT NO: |
PCT/KR08/01158 |
371 Date: |
May 28, 2010 |
Current U.S.
Class: |
428/332 ;
427/249.16; 427/255.394 |
Current CPC
Class: |
C04B 35/14 20130101;
C04B 41/5059 20130101; C04B 41/009 20130101; Y10T 428/26 20150115;
C04B 41/5059 20130101; C04B 41/009 20130101; C04B 35/565 20130101;
C04B 41/87 20130101; C04B 35/522 20130101; C04B 35/522 20130101;
C04B 41/5066 20130101; C04B 41/4556 20130101; C04B 41/4529
20130101 |
Class at
Publication: |
428/332 ;
427/249.16; 427/255.394 |
International
Class: |
B32B 9/00 20060101
B32B009/00; C23C 16/32 20060101 C23C016/32; C23C 16/34 20060101
C23C016/34 |
Claims
1. A method of modifying a graphite substrate, comprising: adding
silicon (Si) powder and silicon dioxide (SiO.sub.2) powder into a
high-temperature reaction chamber holding a bare graphite
substrate; and heating the high-temperature reaction chamber at a
temperature of 1400.about.1600.degree. C. for 3 to 9 hours while
supplying an inert gas and hydrogen to form a 50 .mu.m to 1000
.mu.m thick SiC coating layer on the surface of the graphite
substrate.
2. A method of modifying a graphite substrate comprising: adding
silicon (Si) powder and silicon dioxide (SiO.sub.2) powder to a
high-temperature reaction chamber holding a bare graphite
substrate; and heating the high-temperature reaction chamber at a
temperature of 1450.about.1650.degree. C. for 3 to 9 hours while
supplying N.sub.2 gas to form a 30 .mu.m to 600 .mu.m thick
SiC/Si.sub.3N.sub.4 coating layer on the surface of the graphite
substrate.
3. The method according to claim 1, wherein the bare graphite
substrate has a porosity in the range of 5.about.20%.
4. A graphite substrate modified by the method according to claim
1.
Description
TECHNICAL FIELD
[0001] The present invention relates to a carbon material, and more
particularly to a carbon material useful for engineering components
of electrical contacts, heat exchangers, rockets, aircraft, and the
like.
BACKGROUND ART
[0002] Carbon materials are attractive materials for high
temperature applications due to their high strength, high modulus,
excellent thermal shock resistance, and light weight. They are
widely used as engineering materials, such as heaters, electrical
contacts, high-temperature heat exchangers, rocket nozzles and
leading edges of aircraft wings, etc. Among various carbon
materials, graphite is the most widely used.
[0003] However, the use of graphite materials has been greatly
restricted due to their poor resistance to oxidation at high
temperature in an oxidizing atmosphere. Achieving good oxidation
resistance is essential to utilization of their full potential as
high-temperature materials.
[0004] Prevention of oxidation of graphite materials has been
extensively studied in the past 60 years. Ceramic coatings are
commonly employed to protect graphite materials from oxidation.
Although several coating systems have been developed, silicon
carbide (SiC) is considered as the best coating material due to its
good mechanical properties, low density, good physical-chemical
compatibility with graphite, and excellent oxidation resistance at
high temperature. Also, silicon nitride (Si.sub.3N.sub.4) coating
is of great scientific and technological interest because of good
wear resistance, high hardness, chemical inertness, and excellent
oxidation resistance at high temperature thereof. These properties
allow SiC and Si.sub.3N.sub.4 coatings to meet the conditions
required for a variety of applications, and the SiC and
Si.sub.3N.sub.4 coatings are considered to be the most efficient
method to overcome the shortcoming of the graphite materials.
[0005] In general, the SiC coating is formed by the reaction-formed
process. In the reaction-formed process, molten silicon reacts with
carbon atoms of the graphite substrate to form an SiC coating.
Additional processes such as chemical vapor deposition (CVD) and
chemical vapor reaction (CVR) are also used to form a ceramic
coating on the graphite substrate.
[0006] A solid-vapor reaction (SVR) process for forming a ceramic
coating layer on the carbon material is a modified CVD process,
whereby the surface of the substrate is activated to form a
heat-resistant ceramic coating. The SVR process is advantageous
over other processes in that a uniform coating can be obtained at
low cost. However, this technique provides only a limited coating
thickness, owing to the diffusion barrier, and makes only a single
phase ceramic layer.
[0007] As such, the ceramic coating formed by the conventional SVR
process contributes little to the improvement of the mechanical
properties of the substrate. Especially, there is little systematic
work related to the mechanical properties, such as hardness and
wear resistance, thereabout.
DISCLOSURE
Technical Problem
[0008] Therefore, the present invention has been made in view of
the above problems, and it is an object of the present invention to
provide a method of modifying a graphite substrate by forming a
ceramic coating layer having superior resistance to thermal
oxidation and superior mechanical properties on the graphite
substrate. Particularly, the present invention provides a method of
modifying the graphite substrate by forming SiC and/or
SiC/Si.sub.3N.sub.4 coating layers with sufficient thickness, in
order to improve hardness and wear resistance of the coating
layers.
Technical Solution
[0009] In accordance with an aspect of the present invention, the
above and other objects can be accomplished by the provision of a
method of modifying a graphite substrate by forming an SiC coating
layer on the surface of the graphite substrate. Silicon (Si) powder
and silicon dioxide (SiO.sub.2) powder are added to a
high-temperature reaction chamber holding a bare graphite
substrate. Then, while supplying an inert gas and hydrogen, the
high-temperature reaction chamber is heated at a temperature of
1400.about.4600.degree. C. for 3 to 9 hours to form a 50 .mu.m to
1000 .mu.m thick SiC coating layer on the surface of the graphite
substrate.
[0010] In accordance with another aspect of the present invention,
a method of modifying a graphite substrate by forming an
SiC/Si.sub.3N.sub.4 coating layer on the surface of a graphite
substrate is provided. Si and SiO.sub.2 powders are added to a
high-temperature reaction chamber holding a bare graphite
substrate. Then, while supplying N.sub.2 gas, the high-temperature
reaction chamber is heated at a temperature of
1450.about.1650.degree. C. for 3 to 9 hours to form a 30 .mu.m to
600 .mu.m thick SiC/Si.sub.3N.sub.4 coating layer on the surface of
the graphite substrate.
[0011] The bare graphite substrate may have a porosity in the range
of 5.about.20%.
[0012] In accordance with a further aspect of the present
invention, a graphite substrate modified by the aforementioned
modification methods is provided.
ADVANTAGEOUS EFFECTS
[0013] Through an SVR process with optimized reaction conditions,
SiC and SiC/Si.sub.3N.sub.4 coating layers having superior
mechanical properties are formed on the graphite substrate. The
inventors have succeeded in forming a multi-phase coating layer by
optimizing porosity of graphite, reaction temperature and dwell
time. Consequently, mechanical properties such as hardness and wear
resistance, and resistance to thermal oxidation were significantly
improved. Specifically, hardness of the SiC coating layer increased
to 10.about.15 times that of the graphite substrate. The
SiC/Si.sub.3N.sub.4 coating layer exhibited higher hardness than
the SiC coating layer, although it was thinner. Resistance to
thermal oxidation was significantly improved, as evidenced by the
following. Weight loss of the graphite coated according to the
present invention at high temperature decreased significantly as
compared to bare graphite. Particularly, the SiC/Si.sub.3N.sub.4
coating exhibited superior resistance to thermal oxidation.
DESCRIPTION OF DRAWINGS
[0014] FIG. 1 is a graph depicting changes of free energy for
possible chemical reactions related to the formation of coating
layers, depending on reaction temperature;
[0015] FIG. 2 shows XRD patterns of SiC coatings formed on graphite
substrates with different porosities of (A) 10% and (B) 13% for
dwell time of 6 hours under Ar/H.sub.2 atmosphere at different
temperatures of (a) 1400.degree. C., (b) 1450.degree. C., and (c)
1500.degree. C.;
[0016] FIG. 3 shows XRD patterns of SiC coatings at different dwell
times;
[0017] FIG. 4 shows XRD patterns of SiC/Si.sub.3N.sub.4 coatings at
different dwell times;
[0018] FIG. 5 shows cross-sectional electron micrographs of SiC
coatings at different reaction temperatures;
[0019] FIG. 6 shows cross-sectional electron micrographs of
SiC/Si.sub.3N.sub.4 coatings at different dwell times;
[0020] FIG. 7 is graphs depicting hardness values of SiC coatings
formed on graphite substrates with different porosities of (A) 10%
and (B) 13% at different reaction temperatures;
[0021] FIG. 8 is graphs depicting hardness values of SiC coatings
formed on graphite substrates with different porosities of (A) 10%
and (B) 13% with different dwell times;
[0022] FIG. 9 is graphs depicting hardness values of
SiC/Si.sub.3N.sub.4 coatings formed on a graphite substrate with a
porosity of 10% with different dwell times;
[0023] FIG. 10 is graphs for comparing acoustic emission counts
during scratch tests on SiC coatings formed on graphite substrates
and on bare graphite substrates;
[0024] FIG. 11 is graphs for comparing the results of heat
treatment tests performed on bare graphite substrates, SiC coated
graphite substrates, and SiC/Si.sub.3N.sub.4 coated graphite
substrates at 800.degree. C.;
[0025] FIG. 12 is surface micrographs of bare graphite substrates
before and after oxidation tests at 800.degree. C. for 150
minutes;
[0026] FIG. 13 is surface and cross-sectional micrographs of SiC
coated graphite substrates (A-1 and B-1) before oxidation tests,
(A-2 and B-2) after oxidation tests at 800.degree. C. for 150
minutes, and (A-3 and B-3) after oxidation tests at 1100.degree. C.
for 150 minutes; and
[0027] FIG. 14 is surface and cross-sectional micrographs of
SiC/Si.sub.3N.sub.4 coated graphite substrates (A-1 and B-1) before
oxidation tests, (A-2 and B-2) after oxidation tests at 800.degree.
C. for 150 minutes, and (A-3 and B-3) after oxidation tests at
1100.degree. C. for 150 minutes.
BEST MODE
[0028] According to the present invention, SiC and Si.sub.3N.sub.4
coatings are formed on a graphite substrate by a SVR process, in
which carbon atoms of the graphite substrate are reacted directly.
Microstructure, elemental distribution, hardness, and wear
resistance of the synthesized coating layers depending on reaction
conditions were investigated. Specifically, effects of graphite
porosity, reaction gas, reaction temperature, and dwell time on
microstructural evolution and mechanical properties were
investigated.
Mode for Invention
Synthesis of SiC and SiC/Si.sub.3N.sub.4 Coating Layers on Graphite
Substrates
[0029] Two kinds of graphite substrates with different porosities
of 10% and 13% were cut from 2D-graphite. Graphite specimens with a
size of 10.times.10.times.10 mm were used as the substrates.
[0030] Before coating, the substrates were polished using a 3 .mu.m
diamond paste, and then ultrasonically washed (Sonifier 450,
Branson, VWR Scientific Co., USA) in isopropyl alcohol for 10
minutes. Silicon (Si, Daejung Chemicals & Metals Co., Ltd.,
Korea) and silicon dioxide (SiO.sub.2, Junsei, Tokyo, Japan)
powders for generating SiO vapor were mixed in a molar ratio of
1:1.
[0031] The mixed powders and the substrates were kept in an alumina
crucible and then heated to generate vapor at different
temperatures with different dwell times at a heating rate of
5.degree. C./min in an Ar/H.sub.2 (160:40) flow of 200 ml/min for
SiC coating and N.sub.2 flow of 200 ml/min for SiC/Si.sub.3N.sub.4
coating.
[0032] Structural analysis of the synthesized materials was carried
out by an X-ray diffraction (XRD, Philips X-pret MPD, Model PW3040,
Eindhoven, the Netherlands) employing Cu--K.alpha. radiation.
Microstructure of the synthesized materials was observed by a
scanning electron microscopy (SEM, JEOL Model JMS-840, Tokyo,
Japan) and line spectrums were analyzed using an energy dispersive
X-ray spectrometer (EDS, 52700, Hitachi, Japan).
[0033] FIG. 1 is a graph depicting changes of free energy for
possible chemical reactions related with the formation of coating
layers, depending on reaction temperature. Referring to FIG. 1, the
free energy for generation of SiO vapor decreases with increasing
temperature, whereas the free energy for generation of SiC and
Si.sub.3N.sub.4 increases with increasing temperature.
[0034] FIG. 2 shows XRD patterns of SiC coatings formed on graphite
substrates with different porosities of (A) 10% and (B) 13% for
dwell time of 6 hours under Ar/H.sub.2 atmosphere at different
temperatures of (a) 1400.degree. C., (b) 1450.degree. C., and (c)
1500.degree. C.
[0035] Referring to FIG. 2, the coating layers mainly consist of
SiC phases and carbon residues at relatively low temperatures of
1400.degree. C. and 1450.degree. C. However, with the increase of
reaction temperature, more SiC layers were formed and all carbon
residues were converted to SiC phases when the temperature reached
1500.degree. C.
[0036] FIG. 3 shows XRD patterns of SiC coatings at different dwell
times. The formation of the coatings was performed on graphite
substrates with different porosities of (A) 10% and (B) 13% for
dwell times of (a) 3 hours, (b) 6 hours, and (c) 9 hours under
Ar/H.sub.2 atmosphere at 1500.degree. C.
[0037] Referring to FIG. 3, the synthesized coating layers consist
of mainly .beta.-SiC of FCC structure with a very small amount of
pseudo .alpha.-SiC. A small amount of carbon residues were present
when dwell time was 3 hours, but, with the dwell time increasing to
6 hours, there remained no carbon residues. This is caused by the
fact that the diffusion rate of Si into graphite is slow, and a
long dwell time is thus needed to obtain a thick and pure SiC
coating. However, in the case of a longer dwell time of 9 hours,
carbon peaks appeared again in both XRD patterns, owing to
decomposition of the synthesized SiC.
[0038] Accordingly, in order to synthesize a pure SiC coating layer
with as few carbon residues as possible, optimization of the
reaction conditions including reaction temperature and dwell time,
independent of the porosity of graphite, is necessary. It is
assumed that an insignificant difference found in either XRD
pattern was caused by a very small degree of crystallization.
[0039] However, it is easier to synthesize an SiC coating layer on
the graphite with a porosity of 13% than 10%, because the increase
of cumulative contact area is larger at 13% porosity.
[0040] The SiC coating is synthesized via SVR as follows:
Si(solid)+SiO.sub.2(solid).fwdarw.2SiO(vapor) (1)
SiO(vapor)+2C(from graphite).fwdarw.SiC(solid)+CO(vapor) (2)
[0041] First, the SiO vapor is generated from the powder mixture of
Si and SiO.sub.2 and reacts with carbon (C) to form SiC on the
surface of the graphite, and then diffusion occurs, growing the SiC
layer into the graphite.
[0042] In FIG. 4, XRD patterns of SiC/Si.sub.3N.sub.4 coatings
synthesized on graphite substrates with different porosities of (A)
10% and (B) 13% and different dwell times of (a) 3 hours, (b) 6
hours and (c) 9 hours at 1550.degree. C. under N.sub.2 atmosphere
are shown.
[0043] Referring to FIG. 4, more SiC and Si.sub.3N.sub.4 were
formed on the graphite with a porosity of 13% than of 10%. The
results indicate that more SiC and Si.sub.3N.sub.4 coating layers
are synthesized on the graphite with a porosity of 13% because it
passes a larger amount of SiO vapors to react with carbon in the
graphite. The formation of the SiC phase is similar to that in the
Ar/H.sub.2 atmosphere. N.sub.2 participates in the synthesis of the
Si.sub.3N.sub.4 coating layer, as follows:
3SiO(vapor)+2N.sub.2(vapor)+3C(from
graphite).fwdarw.Si.sub.3N.sub.4(solid)+3CO(vapor) (3)
[0044] FIG. 5 shows cross-sectional micrographs of SiC coatings at
different reaction temperatures. The coatings were formed on
graphite substrates with different porosities of (A) 10% and (B)
13% under Ar/H.sub.2 atmosphere with a dwell time of 6 hours, at
(A-1 and B-1) 1400.degree. C., (A-2 and B-2) 1450.degree. C., and
(A-3 and B-3) 1500.degree. C.
[0045] In FIG. 5, the SiC coating layer is shown in white, and the
graphite is shown in gray. Comparing (A-1) to (A-3), and (B-1) to
(B-3), the SiC layer became thicker and denser as the synthesis
temperature increased. The thickness of the SiC coating layer on
the graphite with a porosity of 13% is thicker than on the graphite
with a porosity of 10%. Also, some SiC phases were grown into the
graphite along the pores, which is mainly caused by SiO vapors
diffusing into the inside surface of the graphite pores.
[0046] The synthesized SiC coating layer adheres well to the
graphite substrate and no cracks are formed therebetween. However,
the thickness is not much affected by the dwell time. Therefore, it
can be confirmed that the thickness of the coating layer is mainly
affected by the porosity and the synthesis temperature. The
thicknesses of coatings are about 200 .mu.m and 400 .mu.m for
substrates with porosities of 10% and 13%, respectively.
[0047] From EDS analysis, it was found that the SiC layer on the
graphite with a porosity of 13% provides a gradual change in Si
distribution. This indicates that the diffusion rate of Si into the
graphite is not sufficiently high. As a result, the pores inside
the graphite mainly affect formation of SiC in the graphite
substrate.
[0048] FIG. 6 shows cross-sectional micrographs of
SiC/Si.sub.3N.sub.4 coatings at different dwell times. The coatings
were formed on graphite substrates with different porosities of (A)
10% and (B) 13% under N.sub.2 atmosphere at 1500.degree. C. with
dwell times of (A-1 and B-1) 3 hours, (A-2 and B-2) 6 hours and
(A-3 and B-3) 9 hours.
[0049] In the micrographs of FIG. 6, the SiC/Si.sub.3N.sub.4
coating layer is shown in white, and the graphite is shown in gray.
Referring to (A-3) and (B-3) of FIG. 6, the thickness of the
SiC/Si.sub.3N.sub.4 coatings is about 50 .mu.m and 100 .mu.m for
the substrates with porosities of 10% and 13%, respectively. That
is, the SiC/Si.sub.3N.sub.4 coating layers synthesized under
N.sub.2 atmosphere are thinner than those synthesized under
Ar/H.sub.2 atmosphere. Under N.sub.2 atmosphere, the thickness of
the SiC/Si.sub.3N.sub.4 coating layers did not increase
significantly even when the reaction temperature was raised to
1600.degree. C. There were some SiC/Si.sub.3N.sub.4 phases along
the pores inside the graphite substrate. The dwell time has little
effect on the thickness of the coating layers. The dwell time only
affected the densification of the coating layers.
[0050] Comparing FIG. 5 to FIG. 6, it can be seen that it is easier
to form thick SiC coating layers on the graphite under Ar/H.sub.2
atmosphere than to form thick SiC/Si.sub.3N.sub.4 coating layers
under N.sub.2 atmosphere. Even when the synthesis temperature is
increased to 1550.degree. C., it is difficult to synthesize thick
SiC/Si.sub.3N.sub.4 coatings on the graphite. This indicates that
N.sub.2 atmosphere restricts the reaction of the SiO vapors and
carbon in the graphite substrate, and the growth of the
Si.sub.3N.sub.4 coating layers. Thus, the increase of synthesis
temperature under N.sub.2 atmosphere seems to enhance not only the
growth of the coating layers but also the breakdown of the coating
layers via the conversion mechanism of Reaction 3.
[0051] The formation and growth of SiC and SiC/Si.sub.3N.sub.4
coatings on the graphite can be explained by the following steps:
i) SiO vapor is generated from the reaction of the mixed Si and
SiO.sub.2 powders; ii) SiO vapor diffuses into the gas phase; iii)
SiO vapor and carbon of the substrate surface react to form SiC;
iv) C and Si diffuse into the SiC layer along the SiC phase
boundaries; and v) C and Si react to form and grow SiC on the
internal interface. Finally, under N.sub.2 atmosphere, Reaction 3
will occur to form Si.sub.3N.sub.4 on the surface of substrate.
[0052] Mechanical Properties of Coating Layers
[0053] The specimens selected for hardness measurement were
sectioned selectively, ground to a 10 .mu.m finish, and then
polished to a 1 .mu.m finish. The top surface was lightly polished,
and finished using a 1 .mu.m diamond paste before scratch tests
were carried out. Ultra-micro Vickers indentation tests (MZT-511,
Mitutoyo, Japan) and scratch tests (UMT, Center for Tribology Inc.,
USA) were conducted to examine the mechanical properties.
[0054] FIG. 7 is graphs depicting the hardness values of SiC
coating layers synthesized on graphite substrates with different
porosities of (A) 10% and (B) 13% with a dwell time of 6 hours at
different temperatures, and displacement versus force curves of
coating layers obtained at 50 .mu.m from the surface.
[0055] Referring to FIG. 7, it can be seen that the hardness of the
synthesized coating layer is proportional to the reaction
temperature. The hardness of the coating layer synthesized at
1400.degree. C. was similar to that of the bare graphite. However,
the hardness of the coating layer was higher when synthesized at
1500.degree. C., which is about 10.about.15 times that of bare
graphite. Specifically, the hardness values of the coatings
synthesized at 1500.degree. C. with a dwell time of 6 hours were
630 HVU for the graphite of 10% porosity and about 400 HUV of 13%
porosity.
[0056] FIG. 8 is graphs depicting the hardness values of SiC
coating layers synthesized on the substrates with different
porosities of (A) 10% and (B) 13% and with different dwell times at
1500.degree. C., and displacement versus force curves of coating
layers obtained at 50 .mu.m from the surface.
[0057] Referring to FIG. 8, it can be seen that the hardness of the
coating layer formed on the graphite with a porosity of 10% is
higher than that formed on the graphite with a porosity of 13%.
This is because the substrate with a porosity of 10% is denser and
the coating layer formed on the graphite with a porosity of 13% has
more pores in the coating and substrate.
[0058] The hardness of the coating layer formed on the graphite
with 10% porosity changes suddenly, whereas that of the graphite of
13% porosity changes gradually from the surface of the graphite
into the graphite. When the dwell time was increased to 9 hours,
the hardness of both coating layers decreased.
[0059] FIG. 9 is graphs depicting the hardness values of
SiC/Si.sub.3N.sub.4 coating layers formed on the graphite with a
porosity of 10% with different dwell times at 1550.degree. C. under
N.sub.2 atmosphere, and displacement versus force curves of the
coating layers obtained at 25 .mu.m from the surface. The hardness
of the coating layer is obviously affected by the dwell time.
Specifically, the hardness increases quickly from 200 to 800 HUV
when the dwell time is increased from 3 to 6 hours. However, there
is little increase in hardness when the dwell time is increased
from 6 to 9 hours, showing about 930 HUV at 9 hours.
[0060] Under N.sub.2 atmosphere, the loose coating layer formed
initially is concentrated into a relatively dense layer as the
dwell time is increased. Compared with the hardness of coating
layer is formed on the graphite with 13% porosity under Ar/H.sub.2
atmosphere, the hardness of the coating layer synthesized on the
graphite with 10% porosity under N.sub.2 atmosphere is about two
times higher than that synthesized under Ar/H.sub.2 atmosphere,
even though the thickness of the coating layer is much thinner than
that formed under Ar/H.sub.2 atmosphere.
[0061] FIG. 10 is graphs for comparing acoustic emission counts
during scratch tests on SiC coatings formed on graphite substrates
and on bare graphite substrates. The SiC coatings were synthesized
on graphite substrates with different porosities of (A) 10% and (B)
13% at 1500.degree. C. with a dwell time of 6 hours. Bare graphite
substrates had different porosities of (C) 10% and (D) 13%.
[0062] Referring to FIG. 10, it can be seen that acoustic emission
signals increase with increase of scratching time, which
corresponds to increase of applied load. A more pronounced number
of acoustic emission signals appeared in the graphite with 13%
porosity than 10% porosity, whereas the critical load of the
coating layer was about 22 N in both cases.
[0063] Both coating layers had the same friction coefficient of
about 0.7 in spite of different porosities of the substrates. This
clearly indicates that the wear resistance of the graphite
substrate is improved by the coating layer, and the porosity of the
graphite substrate does not affect the critical load of the coating
layer, whereas the critical load and the friction coefficient of
the graphite substrate without a coating layer are strongly
affected by the porosity.
[0064] Oxidation Resistance of SiC and SiC/Si.sub.3N.sub.4 Coated
Graphite Substrates
[0065] Thermal oxidation tests were performed at 800.degree. C. or
more for three samples of bare graphite, SiC coated graphite, and
SiC/Si.sub.3N.sub.4 coated graphite.
[0066] A crucible was heated to a predetermined temperature and
then the sample was placed in the crucible. Inside the crucible,
one side of the sample was exposed to high temperature and the
other side was air-cooled to simulate a real application
environment. After the oxidation test, the sample was removed and
cooled to room temperature. The sample was weighed using a balance
with a sensitivity of 0.01 mg.
[0067] The surface of the coating layers was observed before and
after the oxidation tests with the SEM. Cross-sectional
microstructures were also observed.
[0068] FIG. 11 is graphs for comparing the results of heat
treatment tests performed on bare graphite substrates, SiC coated
graphite substrates, and SiC/Si.sub.3N.sub.4 coated graphite
substrates at 800.degree. C.
[0069] As can be seen from FIG. 11, the weight loss of bare
graphite reached 80% when it was exposed to 800.degree. C. for up
to 150 minutes. The weight loss decreased significantly in SiC
coated graphite substrate (10%) and there was no weight loss in
Si.sub.3N.sub.4 coated graphite substrate.
[0070] FIGS. 12 to 14 are surface micrographs of bare graphite
substrates and coated graphite substrates with porosities of (A)
10% porosity and (B) 13% before and after thermal oxidation tests.
Specifically, FIG. 12 is surface micrographs of bare graphite
substrates before and after thermal oxidation tests at 800.degree.
C. for 150 minutes. FIG. 13 is surface and cross-sectional
micrographs of SiC coated graphite substrates (A-1 and B-1) before
oxidation tests, (A-2 and B-2) after oxidation tests at 800.degree.
C. for 150 minutes, and (A-3 and B-3) after oxidation tests at
1100.degree. C. for 150 minutes. In addition, FIG. 14 is surface
and cross-sectional micrographs of SiC/Si.sub.3N.sub.4 coated
graphite substrates (A-1 and B-1) before oxidation tests, (A-2 and
B-2) after oxidation tests at 800.degree. C. for 150 minutes, and
(A-3 and B-3) after oxidation tests at 1100.degree. C. for 150
minutes.
[0071] Referring to FIG. 12, which shows thermal oxidation test
results for bare graphite, there was a significant change in
surface morphologies before and after the tests. It is believed
that such a significant change in surface morphologies is caused by
expansion of the pores present on the surface and inside the
substrate via thermal oxidation. It is consistent with the
observation that the expansion of pores is more prominent in bare
graphite sample with 13% porosity than 10% porosity.
[0072] Referring to FIGS. 13 and 14, expansion of pores was
observed on the surface of SiC or SiC/Si.sub.3N.sub.4 coated
graphite substrates after thermal oxidation at 800.degree. C.
However, change in surface morphologies was not so prominent as in
bare graphite (FIG. 12). Interestingly, pores disappeared from the
surface of the SiC or SiC/Si.sub.3N.sub.4 coated graphite
substrates when the thermal oxidation temperature was increased
from 800.degree. C. to 1100.degree. C., differently from the bare
graphite samples.
[0073] The thickness of the SiC coating layer was much thicker than
that of the SiC/Si.sub.3N.sub.4 coating layer, but the
SiC/Si.sub.3N.sub.4 coated sample exhibited better resistance to
thermal oxidation in the thermal oxidation tests. This result was
consistent in both graphite substrates of 10% and 13%
porosities.
INDUSTRIAL APPLICABILITY
[0074] The present invention provides a method of modifying a
graphite substrate comprising forming a ceramic coating layer, and
is applicable to the production of carbon materials.
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